This invention relates to heat transfer presses, and more particularly to an automatic heat transfer press for tubular structures and containers.
Printed cups, mugs, glasses and other tubular structures (hereinafter collectively "mugs") have become increasingly popular over the past few years, especially as a marketing and advertising tool. There are three basic approaches to printing onto mugs: decal printing, screen printing, and heat transfer.
Decal printing, especially glass decal printing, involves a glass frit arranged onto a decal to form the desired print. The decal is then pressed against the glass or ceramic surface to be imprinted and both the decal and glass placed in an oven. The temperature is gradually increased until the temperature reaches 900 to 1900 degrees F. The process to prepare the decal takes approximately one day for each color and several hours to "fire" the decal onto the mug. Built up color printing is possible, but full color printing is not possible. Whereas in full color printing primary colors can be mixed to attain the desired color, built color printing requires the initial use of the desired end color, the mixing of colors cannot be used.
With screen and pad printing, special glass/ceramic inks can be applied to the surface of the ceramic/glass surface to be imprinted, normally in one color, although two or three colors are possible where close registration is not critical. The printed item is then placed in an oven and the temperature is gradually increased to 900 degrees F. The heating process takes several hours. Generally "firing" takes place after each color on multi-colored designs.
Heat transfer printing is the printing of sublimation transfers onto mugs by heating. The heat transfer process involves transferring sublimation transfers by heat and contact pressure. There are many types of sublimation transfers that can be imprinted. Copy machines can produce a sublimation transfer; video printers can generate a sublimation transfer; laser printers, printing presses, etc. The key to all these images is that they all use a form of "sublimation" ink. The "sublimation" transfer is made up of two basic parts: the transfer release paper and the sublimation dyes. The sublimation dyes are printed onto the transfer release paper. The heat transfer process heats the transfer paper and sublimation dye to a certain temperature. As the temperature of the mug rises during the cycle time, the sublimation dyes start to release from the transfer paper and are transmitted to the coating on the mug. This transitiveness of sublimation dyes from the transfer paper to the coated mug is the key to any heat transfer process. The different types of sublimation transfers work best at different operating temperatures. For example, video processes and films require lower temperatures during the transfer process.
Decal printing is impractical and inflexible for point of sale applications in that the decal printing must be done when the mug itself is fired. Although screen printing has been the historic method of printing onto coffee mugs, the process and equipment required makes it very difficult to print a mug at a point of sale. Heat transfer printing overcomes the limitations of screen and decal printing in that printing may be done at a point of sale, quickly and flexible. Heat transfer printing with sublimations can produce inexpensive "one-of-a-kind" items. The market for cylindrical shaped glass and ceramic products, such as coffee mugs and glasses, lends itself to the one of a kind market. It is also possible to produce full color reproductions of full color designs. The time required to impart a design using heat transfer sublimation can be a matter of seconds and minutes versus hours or days with other technologies.
With heat transfer printing there are several major factors that determine the quality of printing. Among the major variables are: the mug structure, the heat transfer process, mug coating, and transfer placement.
All mugs are different. Each mug has a different wall thickness, ceramic composition, coating, thickness of coating, physical dimensions (inside and outside diameter), slopes, angles, curves, post-curing time of coating, chemical make-up, etc. The differences in mug structures, even two that are nominally alike, must be taken into account by any printing process.
Mug coating plays a major part in the ability to apply a sublimation transfer to a ceramic mug. Ceramic mugs have a hardened layer of material that resists allowing sublimation dyes to impregnate the surface. Mugs to be sublimation printed are coated with a layer of special polymer. These polymers are receptive to sublimation dye, aesthetically acceptable, adhere permanently, and in the case of containers for holding food and beverages, the coating is inert and safe to come in contact with food, skin and may be ingested without causing harm.
Adhering the sublimation transfer to a mug is a critical part of mug printing. Care must be taken to ensure that the transfer paper is tight.
In order to print using sublimation, a properly prepared transfer must be held in tight contact with the receptive surface while heat is applied. The heat and pressure must continue for a sufficient time to allow the sublimation process to complete itself.
The first historic attempts at producing sublimation transfer devices involved the use of cylindrical block heaters. These are heaters which have elements forming two approximate 100 degree arcs about the exterior of the mug. The elements apply heat and pressure to the sublimation transfer to effectuate the imprint transfer. Block heaters have several limitations. The radius of the object to be printed is generally limited to the radius of the heater block. A smaller or larger circumference item does not fit accurately enough into the heat block to produce a uniformly printed surface. Natural irregularities in the surface of the cylindrical container, especially on glass and ceramic objects, create hot spots (places where the pressure is very high) or cold spots (places where the pressure contact is too low). To resolve the latter problem, it is common to place a flexible silicon rubber pad with silicon on the heater block. The rubber pad improves the contact pressure between the heater block and the cylindrical glass or ceramic surface. However, the rubber pad also acts as a heat insulator thereby making it more difficult to attain the needed temperature.
To overcome the limitations of heater blocks heat transfer printing devices for sublimation transfers have been produced using a flexible heater coated with silicon rubber. Such devices allow the mug to be printed up to 300 degrees of the cylinder's circumference. The use of a flexible heater also helps to solve the problem of producing acceptable prints in spite of the natural irregularities in the surface of the glass or ceramic cylindrical surface. The flexible heater also increases the range of cylinder diameters that can be printed.
There are, however, limitations when printing with silicone flexible heaters. The flexible heater has to perform two functions, one of heating and the other of creating uniform contact pressure. The physical properties needed to address the two printing conditions are opposite enough to cause problems. The first problem is that the heater portion of the flexible heater is a fine mesh of conductive resistors which is needed to produce uniform heating over the entire surface. This material is woven and therefore its surface, while flexible on one axis as much as a sheet of typing paper is flexible in one axis, is made of a material hard enough that it imparts its natural weave print onto the printed surface. To eliminate this problem, flexible heaters have been produced that bury the heating unit inside of a silicone rubber material. This eliminates the problem of printing the impression of the woven surface onto the end product's surface, but create new problems. Even though the heater and rubber are flexible by nature, they are produced in a flat state, again much like a sheet of paper, but because of their thickness, which is required to resolve the problem of printing on the naturally irregular surface of glass or ceramic, they do not bend uniformly. The outside of this material sandwich, (which generally measures about 3/8 inch thick), must travel further than the inside surface when being wrapped around an object. The pressure required to wrap and hold the flexible heater against the cylindrical surface to be printed must be applied from the outside, and as a result the inside surface has a tendency to buckle in order to use up the additional material resulting from wrapping this material around a cylinder. This results eventually in wrinkling which is impossible to repair or remove. The wrinkles cause uneven contact pressure to be applied to the cylindrical surface to be printed, thereby resulting in a finished design that reflects the exact shape of the wrinkle. As a result, the flexible heater that develops a wrinkle must be replaced. The wrinkling problem can occur after as few as twenty uses, although it normally lasts for a few hundred uses.
Another problem associated with the flexible heating unit is that the rubber layer and the heating web layer have a tendency to separate after repeated use (as few as fifty, but normally a few hundred uses). This separation creates an electrical shock hazard and further aggravates the problem of surface wrinkles. The heater must be replaced if separation occurs.
A still further problem associated with the prior art flexible heating unit has to do with the natural physical difficulty associated with trying to apply an even pressure between all points of a cylindrical surface and the flexible heating unit. The flexible heating unit wraps around the cylindrical object and applies pressure by pulling from the ends. This tends to create a contact pressure differential at different points about the surface of the cylindrical object. The result is that the print will be darker in areas of high pressure and lighter in the area of low pressure.
Flexible silicon heaters are basically lower temperature devices with external heating devices operating at less than 500 degrees F (Fahrenheit). The temperature limitation is required because silicon breaks down and disintegrates at temperatures approaching 500 degrees F. The lower temperature usually means a longer "dwell" time, i.e., time to transfer the image to the mug. A serious danger with longer dwell times or with high temperatures is that the image being imprinted may burn or yellow.
Prior art heat transfer devices also include devices which heat the inside of a mug with hot air. This is basically a high temperature process. Although effective, these types of devices take from 3 to 4 minutes to accomplish the transfer.
As may be seen in FIG. 9, a conventional ceramic mug 10 has a handle 11, bottom 12, exterior surface 14 and inside opening 13. The mug areas about the handle 11 and bottom 12 are necessarily structurally thicker for support. This results in two "colder" areas on the mug 10 during heat transfer printing. Because of this, prior art heat transfer printing devices have left larger areas around the handle and bottom unprinted or poorly printed. The present invention has a two pronged approach to overcoming the prior art limitations on heat transfer printing.
The press of the present invention allows transfer printing for all transfer types, from processed sublimation and litho transfers to thermal video prints, onto the entire cup--from the very top lip of the cup right to the very bottom and within 1/2 inch of the handle--in less time than of current transfer machines.